This invention relates generally to the field of gas turbine engines, and more particularly to the control of emissions produced by the combustion process within a gas turbine engine.
The products of combustion of fossil fuels include carbon dioxide, carbon monoxide, unburnt hydrocarbons and nitrogen oxide (NOx). Various control schemes and hardware configurations have been used to control the concentration of such emissions while at the same time providing fuel-efficient and stable engine operation. Regulatory changes continue to reduce the allowable level of emissions from electric power generating plants utilizing gas turbine engines. Gas turbine power plants in most locations must now be operated to produce no more than 15 ppm NOx, and in some locations, to produce no more than 7 ppm NOx or even 3.5 ppm NOx. Carbon monoxide emission limits can be as low as 10 ppm. To achieve such low levels of emissions, it is necessary to establish and to maintain very lean combustion conditions. Lean combustion is known to be less stable than rich combustion, and lean-burn combustors are more prone to damaging pressure pulsations generated within the combustor. Precise xe2x80x9ctuningxe2x80x9d of the combustion process is needed to establish a balance between stable combustion and low emissions. A precisely tuned engine may be susceptible to drift over time, with a resulting increase in emissions or an increase in the level of combustion instability.
One known approach to controlling the emissions from a gas turbine power plant is to run the combustor at a relatively rich setting, thereby ensuring stable combustion while generating excessive amounts of undesirable emissions. The exhaust gas is then cleaned to regulatory limits by passing it through a combustion catalyst installed downstream of the combustor in the turbine exhaust system. Alternatively, a catalyst may be used to achieve a majority of the fuel combustion, with only a final portion of the combustion being accomplished in a flame combustor located downstream of the primary catalyst. Catalyst systems are very expensive and are often used as a last resort in especially rigorous regulatory situations.
The generation of NOx emissions is directly related to the peak flame temperature in the combustor. For more than two decades it has been known to control the peak flame temperature in a gas turbine combustor by injecting water into the combustor. The cost of the demineralized water used for water injection can be significant, particularly in areas where the supply of fresh water is limited. Accordingly, it is beneficial to limit the use of injected water to the extent possible. U.S. Pat. No. 4,160,362 dated Jul. 10, 1979, describes a gas turbine power plant having reduced emission of nitrogen oxide. The gas turbine power plant described in that patent includes a system for controlling the amount of water injected into the combustor as a function of gas turbine load corrected for variations in compressor inlet temperature (i.e. ambient temperature) and inlet guide vane position.
An improved apparatus and method are needed to further reduce the level of emissions in a gas turbine engine. For systems utilizing water injection, a reduced quantity of water consumption is desired.
A method is disclosed herein for use with a dual-mode gas turbine engine combustor having a pre-mixed combustion zone and a pilot diffusion combustion zone. The method for controlling the generation of oxides of nitrogen during operation of the combustor includes selectively injecting water into only the pilot diffusion combustion zone and not into the pre-mixed combustion zone to correspondingly limit combustion temperature in the pilot diffusion combustion zone for controlling the production of oxides of nitrogen in the combustor.
A gas turbine engine having a dual-mode combustor is described herein as including: a pre-mixed combustion zone; a diffusion combustion zone; and a water injection apparatus for selectively injecting water into only the diffusion combustion zone and not the pre-mixed combustion zone for controlling combustion temperature in the diffusion flame zone.
A method for controlling combustion of a fuel in a gas turbine engine having at least two zones of combustion is disclosed herein as including selectively injecting a fluid other than the fuel into only one of the at least two zones and not another of the at least two zones. When the gas turbine engine is a dual-mode gas turbine engine combustor having a pre-mixed combustion zone and a pilot diffusion combustion zone, the method further includes injecting the fluid into only the pilot diffusion combustion zone and not the pre-mixed combustion zone.